1. Field of the Invention
This invention relates generally to medical apparatus for monitoring physical parameters such as respiration, blood pressure, temperature, peripheral pulse and heart rate. In particular, this invention relates to such monitors which utilize alarm or threshold circuitry utilized to indicate when the measured parameter does not reach or exceeds a preset minimum or maximum level. These limits may be related to the depth of respiration, the systolic blood pressure, the amplitude of the peripheral pressure pulse or the heart rate.
This invention particularly relates to monitors in which the limit is set proportional to the value of the parameter at a particular time, known as the base period, rather than to an absolute value. That is, during the monitoring of a particular patient, a time interval is chosen in which the operator determines that the measured parameter is considered normal or acceptable and the level is set to a value proportional to the value of the parameter monitored during this base period.
This invention finds particular usefulness in respiration monitors which are used to detect apnea, i.e., the cessation of breathing, especially for use with infants.
2. Description of the Prior Art
In respiration monitors using the impedance pneumographic technique, an electrical signal is applied to the patient, by means of electrodes, to measure the electrical impedance of the patient. It is well known that if the electrodes are properly positioned, this impedance will change as a function of respiration. A voltage signal is thereby developed, the amplitude of which is related to the depth of respiration. A comparator is used to generate a pulse whenever the voltage signal exceeds a predetermined threshold level or limit. This pulse is applied to an elapsed time measuring device which generates the apnea alarm whenever the interval between pulses exceeds a present limit.
The easiest way of providing an adjustable threshold level for use in apnea or other monitors is to utilize a manually adjustable signal source controlled, for example, by an operator-adjusted potentiometer. In this manner an alarm or threshold limit may easily be set to an absolute value, that is, independent of the monitored value of the parameter. It is often more desirable to set the threshold to a value related to the value of the parameter during the base period. That is, it may be desirable to set the threshold level so that the alarm will indicate, for example, if the body temperature rises 0.5.degree. C. or if the amplitude of the peripheral phase drops by two-thirds or if the depth of respiration decreases by 50 percent of the base period value.
It is of particular importance to be able to set apnea alarm limits proportional to the respiration as actually monitored during the base period when using impedance pneumography because interference signals, called artifacts, unrelated to respiration may be relatively large compared to the value of the respiration related signals. Such artifacts make the immediate detection of apnea more difficult. One major cause of these artifacts is the impedance fluctuations generated by pulse waves in large blood vessels as a result of cardiac action.
In fact, clinical investigations have shown that the amplitude of the heart beat artifacts is below 40 percent of the amplitude of the signals caused by respiration if respiration is sufficient and the measuring electrodes are applied at suitable locations, namely in the eighth intercostal space in the anterior or midaxillary line.
The known method for adjusting alarm or threshold levels proportional to the monitored value of the parameter during a base period requires the operator to perform certain manual operations, as follows. The parameter signal is displayed, for example, on a cathode ray oscilloscope. When, in accordance with medical practice, the description of which is not relevant here, the operator determines that the parameter being measured is normal or acceptable, the base period is chosen. The value of the parameter is measured by manually measuring one or more signal amplitudes. The absolute value of this base period signal is calculated. The value of the threshold limit is then calculated to be a value proportional to the value of the base period signal and is applied manually to the monitor. These manual operations and calculations are cumbersome, time consuming, susceptible to error and require relatively skilled personnel.
An improved mechanism for setting an alarm or threshold level proportional to the base period value of the parameter without requiring the use of an oscilloscope is known from Hellige Apnea Monitor, Model No. 236,025.
In this device, during the time selected as the base period by the depression of a key, the operator adjusts a potentiometer to the minimum setting at which all inspirations of sufficient depth just result in the illumination of a trigger control lamp. The resultant potentiometer setting is used as the base period value of the respiration signal and the threshold limit is automatically set to a fixed proportion, i.e., 50 percent, of that value.
It is a primary object of this invention to provide apparatus for automatically and conveniently providing a threshold limit the magnitude of which has a defined relationship to the value of the monitored parameter at the time the threshold limit was set, i.e., during the time chosen as the base period.